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Color Basics

Color can be thought of as a
psychological and physiological response to light waves of a specific frequency
or set of frequencies impinging upon the eye.

Our perception of color arises from the composition of
light - the energy spectrum of photons - which enter the eye. Our eyes are
sensitive to a very narrow band of frequencies within the enormous range of
frequencies of the electromagnetic spectrum.

This narrow band of frequencies is referred to as the
visible light spectrum. Visible light - that which is detectable by the human
eye - consists of wavelengths ranging from approximately 780 nanometer (7.80 x
10-7 m) down to 390 nanometer (3.90 x 10-7 m). Specific
wavelengths within the spectrum correspond to a specific color based upon how
humans typically perceive light of that wavelength. The long wavelength end of
the spectrum corresponds to light which is perceived by humans to be red and the
short wavelength end of the spectrum corresponds to light which is perceived to
be violet. Other colors within the spectrum include orange, yellow, green, blue
and indigo.

The Eye and Color Sensation

An understanding of the human response to color demands that one understand
the biology of the eye. The retina on the inner surface of the back of the eye
contains photosensitive cells. These cells contain pigments which absorb visible
light. Of the two classes of photosensitive cells, rods and cones, only the
cones allow us to distinguish between different colors. The rods are effective
in dim light and sense differences in light intensity. In dim light we perceive
colored objects as shades of grey, not shades of color.

A
cross-sectional representation of the eye showing light entering through the
pupil. The photosensitive cells, cones and rods, are located in the retina:
cones respond to color and rods respond to light intensity.

Cones Red, Blue and Green of the Retina

Color is perceived in the retina by three sets of cones
which are photoreceptors with sensitivity to photons whose energy broadly
overlaps the blue, green and red portions of the spectrum. Color vision is
possible because the sets of cones differ from each other in their sensitivity
to photon energy. The maximum sensitivity is to yellow light, but cone R
has a maximum in the red-orange, G in the green-yellow, and B in
the blue. The sensitivities of the three comes overlap. For every color signal
or flux of photons reaching the eye, some ratio of response within the three
types of cones is triggered. It is this ratio that permits the perception of a
particular color.

The response of the tree cones to incident
light: cone R (pigment R) has a maximum sensitivity in the orange-red, cone G
(pigment G) in the green-yellow, and cone B (pigment B) in the blue portions of
the visible spectrum. The sensitivities of the three cones overlap and the
perceived color is due to the relative response of the three cones.

In the same manner, the green cone is most sensitive to
wavelengths of light associated with the color green; yet the green cone can
also be activated by wavelengths of light associated with the colors yellow and
blue. The graphic is a sensitivity cone which depicts the range of wavelengths
and the sensitivity level for the three kinds of cones. The cone sensitivity
curve shown above helps us to better understand our response to the light which
is incident upon the retina. While the response is activated by the physics
of light waves, the response itself is both physiological and psychological.

The human visual system is
logarithmic and not linear. For example, a beam of light that contains mostly
short-wavelength blue radiation stimulates the cone cells that respond to
430-nanometer light far more than the other two cone types, activating the blue
color pigment, and that light is perceived as blue. Light with a majority of
wavelengths centered around 550 nanometers is seen as green, and a beam
containing mostly 600 nanometer wavelengths or longer is seen as red. Pure cone
vision is referred to as photopic vision and is dominant at normal light levels,
both indoors and out. Most mammals are dichromats, usually only able to
distinguish between bluish and greenish color components. In contrast, some
primates, most notably humans, exhibit trichromatic color vision, splitting the
red, green and blue light stimuli.

Suppose that white light - i.e., light consisting of the full range of
wavelengths within the visible light spectrum - is incident upon the retina.
Upon striking the retina, the physiological occurs: photochemical reactions
occur within the cones to produce electrical impulses which are sent along
nerves to the brain." Upon reaching the brain, the psychological occurs: the
brain detects the electrical messages being sent by the cones and interprets the
meaning of the messages. The brain responds by saying "it is white." For the
case of white light entering the eye and striking the retina, each of the three
kinds of cones would be activated into sending the electrical messages along to
the brain. And the brain recognizes that the messages are being sent by all
three cones and somehow interprets this to mean that white light has entered the
eye.

The Visible Spectrum

Since the colors that compose sunlight or white light have different
wavelengths, the speed at which they travel through a medium such as glass
differs; red light, having the longest wavelength, travels more rapidly through
glass than blue light, which has a shorter wavelength. Therefore, when white
light passes through a glass prism , it is separated into a band of colors
called a spectrum . The colors of the visible spectrum, called the elementary
colors, are red, orange, yellow, green, blue, indigo, and violet (in that
order).

Apparent Color of Objects

Color is a property of light that depends on wavelength. When light falls on an
object, some of it is absorbed and some is reflected. The apparent color of an
opaque object depends on the wavelength of the light that it reflects; e.g., a
red object observed in daylight appears red because it reflects only the waves
producing red light. The color of a transparent object is determined by the
wavelength of the light transmitted by it. An opaque object that reflects all
wavelengths appears white; one that absorbs all wavelengths appears black. Black
and white are not generally considered true colors; black is said to result from
the absence of color, and white from the presence of all colors mixed together.

Figure 5. White light
composed of all wavelengths of visible light incident on a pure blue object.
Only blue light is reflected from the surface.

Additive Colors

Colors whose beams of light in various combinations can produce any of the color
sensations are called primary, or spectral, colors. The process of combining
these colors is said to be "additive" ; i.e., the sensations produced by
different wavelengths of light are added together. The additive primaries are
red, green, and blue-violet. White can be produced by combining all three
primary colors. Any two colors whose light together produces white are called
complementary colors, e.g., yellow and blue-violet, or red and blue-green.

Subtractive Colors

When pigments are mixed, the resulting sensations differ from those of the
transmitted primary colors. The process in this case is "subtractive," since the
pigments subtract or absorb some of the wavelengths of light. Magenta
(red-violet), yellow, and cyan (blue-green) are called subtractive primaries, or
primary pigments. A mixture of blue and yellow pigments yields green, the only
color not absorbed by one pigment or the other. A mixture of the three primary
pigments produces black.